When I reply to an email I received from a craigslist ad I posted, today I receive an email from mail.postmaster.hotmail.com with the subject: Delivery Status Notification (Failure) and the message:
This is an automatically generated Delivery Status Notification.
Delivery to the following recipients failed.
*** Email address is removed for privacy ***It does this for many different addresses at reply.craigslist.org
On another hotmail account sending to these addresses, I get the same undeliverable message, but using my gmail account to email the same addresses the messages are delivered, so the problem is not with craigslist relay servers, it is with outlook.com servers.